1. Field of the Invention
The present invention relates to a wire-wound common-mode choke coil which removes common-mode noise occurring on a transmission line.
2. Description of the Related Art
Common-mode choke coils, for example, are disclosed in Japanese Unexamined Patent Application Publication No. 2003-168611 and Japanese Unexamined Patent Application Publication No. 2000-133522.
The common-mode choke coil includes two wires wound on a winding core portion of a core having flanges at both ends, both ends of the wires being connected to electrodes on the flanges, and a ferrite plate arranged over the upper surfaces of the flanges.
This configuration is capable of removing common-mode noise entering a differential transmission line, for example.
However, the above-mentioned conventional common-mode choke coil has the following problems.
Usually, before products are put on the market, the products are subjected to an immunity test in which the products are exposed to electromagnetic interference to determine whether they resist various types of electromagnetic interference.
In the immunity test for common-mode noise of a common-mode choke coil, the common-mode choke coil is arranged in front of a receiving integrated circuit (IC) connected to a transmission IC through a differential transmission line. In addition, a differential signal is transmitted from the transmission IC to the receiving IC through the differential transmission line, and common noise is produced on the differential transmission line and superimposed on the differential signal. In this state, it is determined whether or not the transmission IC and the receiving IC cause a malfunction.
However, in this immunity test, the inductance of the common-mode choke coil and the input capacitance of the receiving IC define a resonant circuit, and the ratio of suppression of common-mode noise decreases at the resonance frequency of the resonant circuit and in a frequency band near the resonance frequency. In this case, a problem occurs in which the common-mode choke coil does not pass the immunity test due to a malfunction of the transmission IC and the receiving IC.